There is a tendency to employ an electric drive motor as the means for operating a brake actuator for a vehicle. In the present case this electric motor is used for operating a disc brake, especially but not exclusively for a heavy road vehicle, such as a truck, trailer, or bus. This disc brake contains means for transforming the rotational movement of the electric motor into a translational movement of a brake pad.
It is advantageous to use an electric motor with a high rotational speed, which has to be reduced. This rotational speed reduction—and accordingly moment increase—can be accomplished by means of a conventional gear train, but a planetary gear unit may provide advantages, for example with regard to volume and cost.
A planetary gear unit falling within the definition above is shown in EP-A-0 372 219. In its portion falling within said definition, this planetary gear unit is per se conventional and is not shown and described in any detail.
An important aspect is that the planetary gear unit shall be as compact as possible without in any way sacrificing its function and durability.